The invention relates to a method for the preparation an active Ru catalyst solution for the transvinylation of carboxylic acids and use thereof in the transvinylation of carboxylic acids.
The transvinylation of carboxylic acids serves to produce vinyl esters. This is understood to mean the transfer of a vinyl unit of a reactant vinyl ester (1V) to a reactant carboxylic acid (2S) to generate a product vinyl ester (2V) and the corresponding acid of the reactant vinyl ester (1S).

The transvinylation of vinyl esters with carboxylic acids in the presence of palladium catalyst is known from EP 376075 B1, in which copper bromide and especially lithium compounds are used as cocatalysts.
In addition to palladium catalysts and mercury catalysts, ruthenium compounds are also used as catalyst in the prior art for transvinylation of vinyl esters with carboxylic acids. Ruthenium compounds are characterized by their high solubility, low volatility and high thermal stability. In addition, they have high, temperature-inducible activity.
A method for transvinylation of carboxylic acids using various Ru compounds as catalyst precursor is described in EP 351603 A2 (=EP506070, U.S. Pat. No. 4,981,973, U.S. Pat. No. 5,155,253). The authors postulate a [Ru(CO)2RCO2] unit as a critical structural element in the formation of the active species. Consequently, all Ru compounds may be used as catalyst precursors which can be converted in situ into this structural element. When using ruthenium(III) chloride as starting compound, the addition of an alkali metal carboxylate is required to generate the active species. Example 16 describes the transvinylation of benzoic acid (100 mmol) with vinyl acetate (200 mmol) using a RuCl3/sodium acetate mixture, which is converted in situ under the transvinylation conditions to the active catalyst compound. After a reaction time of one hour in the transvinylation at 130° C., a yield of 27% of vinyl benzoate is achieved.
In Adv. Synth. Catal. 2013, 355, 2845-2859 the theory of EP 351603 A2 is confirmed and a [Ru(CO)3(RCO2)2] complex as active catalyst species is postulated. The formation of the catalytically active species takes place by the reaction of RuCl3 with sodium hydroxide, vinyl acetate (reactant vinyl ester) and propionic acid (reactant carboxylic acid). The reaction takes place over 4 hours at a temperature of 140° C. and a molar ratio of vinyl acetate to propionic acid of 2.7:1. The yield of active ruthenium catalyst is stated as 53%.
EP 497340 A2 (U.S. Pat. No. 5,210,207) describes a transvinylation process for preparing product vinyl esters whose boiling points are higher than those of the reactant vinyl esters. By reactive distillation of at least one of the product components, the reaction equilibrium is shifted to the product side. The Ru catalysts described in EP 351603 A2 are preferably used for this purpose. Examples in which the active catalyst species are generated from RuCl3 and are used are not stated. The active Ru catalysts used are Ru carbonyl acetate and Ru dicarbonyl acetate.
A method is described in WO 92/09554 A1 in which the reaction mass is firstly separated after the transvinylation and the product vinyl ester is subsequently separated by azeotropic distillation. This method focuses especially on the separation of acid/vinyl ester mixtures with low boiling point differences. The Ru catalysts from EP 351603 A2 (Ru carbonyl acetate and Ru dicarbonyl acetate) are preferably used in the transvinylation reaction. The use of RuCl3-based catalyst systems is not described in the examples.
WO 2013/117294 A1 describes a continuous method for preparing carboxylic vinyl esters. The transition metal catalysed transvinylation is operated in the steady-state and the reaction mixture is separated in a subsequent step. WO 2013/117295 describes a further configuration of this process with a subsequent derivatization of the resulting conjugate acid of the reactant vinyl ester. In the examples of both documents, Pd catalysts are mainly used for the transvinylation. RuCl3-based catalysts are not described.
The use of Ru catalysts in the transvinylation reaction has distinct advantages compared to Pd catalysts with respect to solubility, volatility, thermal stability and thermally inducible activity. Numerous Ru compounds can be converted in situ to active Ru species which catalyse the transvinylation reaction. The specific preparation of a concentrated active catalyst solution based on industrially available ruthenium halides is not known.
Therefore, the object consisted of developing a method for preparing an active Ru catalyst solution having an Ru concentration of more than 0.5%, which is characterized by a high Ru yield based on the ruthenium halide used and a high activity.